The use of basic instrumentation in private passenger automobiles has been common from the early part of this century when automobiles were first developed which could be driven at a greater rate of speed than it was safe for most drivers to operate them. In modern automobiles, the most basic instrument remains the speedometer which indicates to the driver the rate of speed at which the car is traveling. It is well known that driving at a relatively high speed for extended periods of time gives operators the illusion that their speed is lower than it actually is as they become acclimated to moving at a relatively high speed. This is particularly true on limited access highways.
Use of other automobile instrument indicators is also well known. In more recent years, there has been a growing trend toward standardized symbology. The International Standards Organization (ISO) has adopted a number of standard graphic symbols to indicate certain conditions existing in the automobile, such as headlamps on high beam, low fuel, low oil pressure, and the like. Conventional instrument displays in automobiles are located in an area in front of the driver, below the windshield or windscreen, which is normally referred to as the dashboard or instrument panel. As is well known, this arrangement requires drivers wishing to check their current rate of speed to momentarily lower their eyes so that they are focusing on the speedometer on the dashboard and not watching the road.
An automobile traveling 55 miles an hour is moving at slightly over 80 feet per second, and can travel a considerable distance in the time required for the operator to observe the speedometer and return his or her attention to the road ahead. Particularly under conditions of night driving, with headlamps on low beam, a considerable portion of the illuminated distance in front of the automobile, at the time the driver's attention was diverted to the dashboard, can be covered prior to the driver's visual attention being returned to the road ahead. This can create conditions of some danger under the circumstances described.
The problem is exacerbated in many European countries in which it is common to find drivers traveling on limited access highways at speeds in excess of 100 miles per hour. For example, an automobile traveling at 110 miles an hour is moving at a rate greater than 160 feet per second, and even momentary inattention to the road, particularly at night, can place the driver in circumstances in which a newly-seen danger in the roadway (when the driver's attention returns to the road ahead) cannot be avoided due to the speed of the car and the driver's reaction time.
Therefore, for safety reasons and reasons of general convenience, it has been suggested that a head-up type display, similar to those found in fighter aircraft, be used in the environment of an automobile to provide the driver with a means for reading the automotive instrumentation without removing the driver's visual attention from the roadway. It should be understood that this general suggestion was provided by the well known test pilot, General Chuck Yaeger, who is not a member of the present inventive entity. The suggestion made by General Yaeger was simply that some form of head-up display be applied to the environment of an automobile.
Head-up displays are well known in military aircraft, particularly modern sophisticated fighter aircraft. Generally characterized, a head-up display is a display apparatus which provides a visual representation of data to the pilot of the aircraft which is visually superimposed on the pilot's normal field of view through the aircraft windshield to the outside world. In most fighter aircraft, a cathode ray tube is the source of the visual image for the head-up display. In conventional aircraft head-up displays, a zero power dielectric coated glass surface is included between the location of the pilot's eyes and the aircraft windshield so that the pilot will see both the visual information from the CRT, and can see outside the aircraft. This glass surface is normally referred to as a combiner because it is used to combine visual images from outside the aircraft with those generated by the CRT. The real world overlay positioning of the symbology must be extremely accurate; less than 1 milliradian deviation for military usage. For an automobile this overlay is of minor importance. An optical focusing system which normally includes at least one lens and one mirror focuses information from the CRT onto the combiner glass. The dielectric coating is used to increase the reflectivity so that sufficient light from the CRT image reaches the pilot's eyes.
In addition to reproduction of normal flight instruments, head-up displays in fighter aircraft also include targeting displays and other information for the pilots. In other words, a head-up display may be used to generate what the modern analog of the crosshairs of a gun sight which will be used in aiming missles and the like in combat. Thus, in fighter aircraft head-up displays, the physical location of the CRT images for the head-up display may be critical.
Head-up displays in fighter aircraft were developed, in large part, because of the very high speeds these planes travel. For example, modern fighter aircraft traveling at Mach 2 are traveling in excess of 2200 feet per second which is more than 0.4 miles per second. Needless to say, even momentary inattention to the world outside the airplane, in order to observe an instrument panel, can be very dangerous for the operator of the aircraft.
Because of the types of information displayed in fighter aircraft head-up displays, as described above, such displays are designed to have a large total field of view, a relatively small instantaneous field of view, and thus a relatively small eyebox. In the parlance of such displays, the eyebox is a description of a three-dimensional space in which the viewer's eyes may be located to properly view the display.
Intuitively, the small eyebox resulting from a large total field of view and small instantaneous field of view results in the fact that fighter aircraft head-up displays are normally arranged so that the pilot's eyes are very close to the combiner glass. In most fighter aircraft, there is a relatively large volume of space available within the aircraft in front of the location of the combiner glass. This allows a relatively long straight optical path to be provided between the CRT source of the images for the head-up displays and a mirror which reflects this image up to the combiner glass.
The inventors of the present invention encountered a number of practical problems in attempting to make a straightforward transfer of head-up display technology used in fighter aircraft to the environment of an automobile. In essence, it is impractical to simply install fighter aircraft head-up display technology into the environment of an automobile. Given the suggestion of using a head-up display, such as used in a fighter aircraft in an automobile, it may well be obvious to simply transport the equipment normally constituting the head-up display to the automotive environment. However, this is impractical for a number of reasons. It was the overcoming of the problems encountered, and the impracticality of making the straightforward transfer referred to above, which resulted in the present invention.
Further, some information available at the periphery of the total field of view is seen by the pilot only as he intentionally moves his head from side to side. The net effect is similar to that of looking at a panoramic scene through a knot hole in a fence. This feature is considered desirable in the case of the highly trained pilot; less frequently used symbology information disappears to the side until referenced, reducing clutter in the central image, and lessening any information overload condition the pilot may experience.
First, it is considered impractical from the viewpoint of cost, to simply apply aircraft head-up display technology to the environment of an automobile. The cost of such a head-up display would be an appreciable fraction of the normal cost of the automobile. Thus, the benefits to be derived from a head-up display would not justify the cost of installing such a device.
Secondly, head-up displays of the type used in military aircraft occupy a substantial volume. A straightforward installation of an aircraft type head-up display would require either a large volume to be placed above the normal line of the automobile hood, or breaking through the firewall into the engine compartment in a manner which is deemed to be wholly impractical. Similarly, there is insufficient room between the firewall and the dashboard to allow practical redesign of an otherwise normal passenger automobile to accommodate a conventional head-up display. Thus, application of head-up display technology to the environment of an automobile requires a way to reduce the volume occupied by the display.
Thirdly, the inventors have discovered that to design a practical head-up display for an automobile requires a relatively large eyebox since the physical location of the operator's head with respect to the combiner will vary much more widely in a passenger automobile than in a fighter aircraft. There is a considerable nonuniformity in the height of drivers and thus the vertical position of the driver's head and eyes. Similarly, the great variation in the length of person's legs, and their preference for resting positions for their legs during operation of an automobile has led to design of a wide range of adjustments for the distance from the driver's seat to the dashboard and windshield area of a car.
In contrast to the aviation setting, the automobile driver requires a relatively small amount of information, in a relatively small angular space, which is comfortably visible without head motion to view all the symbology or training in its use. Furthermore, it is believed that a practical head-up display for an automobile should provide a fairly compact instrument cluster which is visible from any area in space in which a driver's eyes might be located. This leads to the need to design a head-up display with a large eyebox determined by an instantaneous field of view which is equal to or exceeds the total field of view of the display.
Again, as a cost consideration, use of a CRT as the image source for the head-up display is impractical. This has led the inventors to the use of a high output light-emitting diode array as described hereinbelow.
Additionally, it should be understood that in some ways the environment of a conventional passenger automobile is more harsh on such a display than the environment of high performance fighter aircraft. In particular, a head-up display is one which includes an optical path from the display source to the combiner which is located at or near the windshield of the vehicle in which the display is installed. While military aircraft are treated with special care while parked on the ground and out of service, it is well known that automobiles are often left standing for extended periods of time in environments which range from extreme cold to extreme heat and sunlight.
The temperature within an automobile parked in the sun on a hot day, can easily exceed 140.degree. F. Furthermore, an optical path from the source, through the exit orifice up to the combiner, normally includes an axis which is substantially perpendicular to the ground. This means that when the sun is overhead, there is a small but finite probability that there may be a direct optical path coupling the sun back to the source of the display image. This can lead to the focusing of very intense visible and infrared light rays at the display image which may damage an LED array of the type employed in the preferred embodiment. Therefore, heat dissipation at the image source of the display, as well as protection from the sunlight becomes an important factor.
All of the foregoing problems have been addressed in the creation of the present invention with respect to automotive head-up displays irrespective of whether a separate combiner is provided.
However, research has shown that operators of automobiles have a negative reaction to a separate combiner glass used in a head-up display in the environment of an automobile. Thus, in creation of a head-up display for an automobile, it is preferred to have the ability to use the conventional automotive windshield as a combiner. This leads to additional problems not present in the design of head-up displays for military aircraft.
First and foremost is the fact that most automotive windshields are not spherical curved surfaces. In modeling the shape of an automotive windshield, it has been found that the regular geometric solid they most closely resemble is the interior surface of a portion of a torus. This leads to two problems in using an automotive windshield in place of a conventional flat combiner. First, the curvature of the windshield gives it a finite focal length and thus the windshield has optical power. Conventional combiners for head-up displays are flat, with a focus at infinity and thus have no optical power.
Secondly, because the windshield approximates the inside surface of a torus, its power is astigmatic. Additional astigmatism occurs due to the angular tilt between the windshield normal and the optical axis of the display unit. Thus, correction for this astigmatism must be added to the optics of the display unit. Some form of predistortion to correct for the astigmatism caused by the reflection of light from the windshield is necessary.
In summary, the present invention was necessitated by the problems encountered in attempting to make a practical implementation of the suggestion that a head-up display of the type used in fighter aircraft be applied to the environment of an automobile.